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Indirect consequences Findings at a 'natural laboratory' on reefs in Papua New Guinea suggest lab experiments have underestimated the impact of rising CO2 on shrimps and crabs.

This is because they were unable to pick up on the indirect effect on animals from destruction of coral habitat due to ocean acidification, says marine ecologist Dr Katharina Fabricius from the Australian Institute of Marine Science in Townsville.

"Our field study shows that ecological outcomes can be quite unpredictable from the physiological lab studies," says Fabricius.

As atmospheric CO2 levels rise more CO2 dissolves in the ocean and leads to acidification that can make it hard for some marine organisms to build their calcium carbonate shells.

Scientists have thought sea urchins, sea snails, sea stars, and mussels would be most vulnerable to this, whereas crustaceans like shrimps and crabs were predicted to be quite CO2 tolerant, says Fabricius.

"This is because of their mobility, active metabolism, ability to buffer body fluid pH, and less problems with shell formation," she says.

"They use mostly chitin and are good in recycling the little carbonate in their shells when they moult."

But, says Fabricius, the laboratory only allows short-term experiments with single species and does not reflect the real-world context animals live in.

Natural laboratory

She and colleagues took advantage of a reef in Papua New Guinea where volcanic CO2 has been seeping into the ocean over a long period of time to see whether such predictions held out.

"Our site is the only site in the tropical Pacific where we can understand the ecosystem effects of ocean acidification," she says. "It's a real natural laboratory."

The researchers had access to a range of different underwater CO2 concentrations and compared the impact of 400 part per million, 700 parts per million and 1000 parts per million CO2 on the abundance of 12 different classes of marine invertebrates.

These different concentrations provided the researchers with a sort of "time machine" to study the impact of ocean acidification on corals and marine invertebrates as CO2 levels rose throughout the century.

Loss of habitat

The researchers found that as CO2 levels rose, the overall abundance of the marine animals fell.

"Basically what we found is that under elevated CO2, the diversity and the density of these organisms was severely reduced," says Fabricius.

Also, she says, as CO2 levels rose, the reefs changed from being dominated by "picture postcard" corals with complex structures, to boulder-type corals that provided less habitat and hiding space for animals.

"Corals are incredibly sensitive to high CO2 so coral communities change," says Fabricius.

To investigate the impact of this change in coral habitat, the researchers took a more fine grained approach.

"We ended up with thousands of quadrants in high and low CO2 environments," says Fabricius.

They found that where there were still complex corals left, there were more animals than when there were only boulder corals.

"For example, physiologists are predicting that the crabs and shrimps are relatively CO2 tolerant from all the lab experiments they've done," she says. "But in our field sites, crabs and shrimps were among the groups that were most severely affected - in this case, by the indirect effects."

Study limitations

Fabricius says there are reasons to think that the impacts seen at the seep site may be less than what will happen under climate change.

In a warming world, oceans will not only be more acidic but also warmer, and this is not reflected by the seep site, says Fabricius.

"I don't think the warming will make it easier for any organism. Cumulative stresses tend to make things much more difficult."

Also, she says the study site was surrounded by healthy coral reefs which could have been providing the seep area with new larvae, preventing a more severe decline in animal numbers.

"So what we're seeing at the seeps may only be a shadow of what's to come for coral reefs," says Fabricius.